In general, the radio-navigation signals transmitted by satellites (or pseudolites) in a positioning system have the form of a carrier modulated by a spreading waveform containing a pseudo-random binary code. The modulation of the carrier causing the spread of the spectrum around the carrier frequency, the radio-navigation signals are often called “spread spectrum” signals. The pseudo-random codes represent an identifier of the signal and, therefore, of the satellite transmitter. Known by the receivers, they allow these a Code-Division Multiple Access (CDMA). Subsidiarily, some satellite positioning signals can also carry useful data (e.g. the navigation message) as a binary sequence (at much lower rate than the pseudo-random code) additionally modulated on the carrier.
In the case of GPS, the radio-navigation signals are transmitted in the frequency bands L1, centred on 1575.42 MHz, and L2, centred on 1227.6 MHz. As part of the modernisation of GPS, the L5 band, centred on 1176.45 MHz, will be added. The Galileo constellation satellites will transmit in the bands E2-L1-E1 (the portion of the median L1 band is the same as that of GPS), E5a (which, according to the Galileo nomenclature, represents the L5 band scheduled for GPS), E5b (centred on 1207.14 MHz) and E6 (centred on 1278.75 MHz). Note that satellites of the Compass constellation transmit or will transmit in the band B1 (centred on 1561.098 MHz), B1-2 (centred on 1589.742 MHz), L1 (centred on 1575.42 MHz), B2 (centred on 1207.14 MHz) and B3 (centred on 1268.52 MHz). The centre frequencies are the carrier frequencies of the various signals.
The reception of a radio-navigation signal typically includes a first demodulation using an internal replica of the carrier generated in the receiver by an oscillator driven by a carrier tracking loop and a second demodulation using an internal replica of the spreading waveform produced by a waveform generator controlled by a spreading waveform tracking loop (also called “code tracking loop”). The control signals of the carrier tracking loop and the spreading waveform are used by the receiver to determine its position. The signal of the phase difference between the signal carrier received and the internal carrier replica produced at each time step by the carrier tracking loop provides a first observable (the phase observable or measurement). The signal delay between the spreading waveform of the received signal and the replica internal spreading waveform produced at each time step by the tracking loop spreading waveform is a second observable (the code observable or measurement).
The elementary measurements that a receiver can make thus include code measurements and carrier phase measurements. These elementary measurements can obviously be combined. The code measurements are accurate to the order of the metre whereas the phase measurements are accurate to a few mm. However, the phase measurements have the disadvantage that they deliver only the real part of the difference in carrier phase between the emission by the satellite and the receiver. The phase measurements are therefore ambiguous in the sense that the number of integer cycles between the transmitter (the satellite) and the receiver is unknown at the start. In order to be able to benefit from the accuracy of the phase measurements, a receiver must resolve the ambiguities by which they are vitiated.
The resolution of the phase ambiguities is commonly done by differentiation of the phase measurements (single or double differentiation). This differentiation enables (not modelled) error sources common to several measurements to be eliminated, and thereby allows an integer information to be revealed, which, when taken into account, further improves performance. However, this integer information consists of the differences between one or more elementary phase ambiguities and does not generally enable one to work back to the elementary phase ambiguities.
Patent application FR 2 914 430 describes a method that solves, in a consistent manner, the phase ambiguities on a network of reference receivers with the aid of dual frequency observations (i.e. code and phase measurements on at least two distinct frequencies). At the same time, this method produces a set of satellite clocks that can be used as assistance data by a dual-frequency receiver external to the network (e.g. that of a user who wishes to know his position). These clocks have the particular property of highlighting entire phase ambiguities when one solves the positioning equations obtained by “elementary” measurements, i.e. neither differentiated between satellites nor between receivers. The disadvantage of the method of the application FR 2 914 430 is that it can only be applied by dual-frequency receivers.